Conductive materials, such as conductive substrates, can be used to build structures with semiconductors to create useful devices. Semiconductors are materials that can contain either an excess of free electrons (N-type) or “holes” (P-type). N- and P-type materials can be joined to form diodes and transistors. Where the two films meet, negative charges can migrate across the junction to the positive side and vice versa, until an equilibrium is reached. This configuration can be used to create light emitting diodes (“LEDs”) or photovoltaic (“PV”) structures.
When a voltage is applied to an LED, a current flows through the junction in the form of electrons moving in one direction while holes move in the other direction. The migration of ionic charge across the junction causes a higher electrical potential than normal, which affects the way electrons combine with holes. When an electron combines with a hole, it can form an excited state pair, called an exciton, which can quickly release the energy as photons of light. Conversely, when a light of an appropriate wavelength is applied to a photovoltaic structure, photons are absorbed by the molecules, form excitons. The charges of the exciton can separate, moving toward opposite electrodes, thereby establishing a current.